The invention relates in general to probers, which are used to ascertain electrical properties of electronic components, referred to hereafter as test substrates in general. The invention particularly relates to a chuck which is used to support and retain a test substrate and, in addition, a calibration substrate. The chuck comprises a first support surface for supporting a test substrate and a second support surface, which is laterally offset to the first support surface, for supporting a calibration substrate, dielectric material or air being situated below the calibration substrate at least in the area of the calibration standard, and the calibration substrate having planar calibration standards for calibrating a measuring unit of the prober.